Ten years ago, one of the greatest scientific discoveries of modern times took place. On September 14, 2015, the LIGO detectors in the states of Louisiana and Washington (USA) struck gold. Both detectors – so-called laser interferometers – picked up extremely small but significant variations in space along their 4-kilometer-long arms. The variations were thousands of times smaller than the diameter of a proton. They were caused by gravitational waves: ripples in the fabric of space and time resulting from extremely violent events in the universe. In this case, the source of the gravitational wave was a merger of two black holes, each several dozens of times more massive than the Sun and located more than a billion light-years away from Earth.
Before merging, the black holes spiralled toward each other, warping space enough to produce a wave detectable here on Earth. They did so with their colossal masses, because according to Einstein’s theory, the gravitational pull of massive moving objects causes the space around them to ripple. The signal picked up by the LIGO detectors showed the final stage of the approach of the black holes as they spun faster and faster around each other. When converted into an audio signal, this spinning could be heard as a “chirp” which lasted only a second. The chirping signal of the first detected gravitational wave – a historic event only announced in February 2016, after physicists were absolutely certain they had indeed measured a gravitational wave – captured the world’s attention. From that moment on, astronomers and cosmologists could not only observe the universe but also listen to it.
